au represents the Top Level Domain for Australia.

Japan (.jp), Germany (.de) are other examples of Top Level Domains. Any site that ends with a .au is a site built specifically for Australians. A top level domain representing a country is called a country code Top Level Domain (ccTLD).

A list of ccTLD's can be found at: http://www.iana.org/cctld/cctld-whois.htm

2LD	Purpose
asn.au	For non-commercial organisations. Includes associations incorporated in any Australian State or Territory, political parties, trade unions, sporting or special interest clubs, charities or non-profit organisations.
com.au	For commercial purposes. Includes commercial entities, currently registered and trading in Australia, as well as commercial products and services.
id.au	For Australian individuals (citizens, residents).
net.au	For commercial purposes. Includes commercial entities, currently registered and trading in Australia, as well as commercial products and services.
org.au	For non-commercial organisations. Includes associations incorporated in any Australian State or Territory, political parties, trade unions, sporting or special interest clubs, charities or non-profit organisations.